The invention relates to a brewing means for a coffee machine of the type having a brewing chamber which is disposed in a housing provided with an opening at one side thereof, a closing element for closing the opening, a hot water line for supplying brewing water under pressure into the brewing chamber and a seal disposed between the closing element and the housing and extending around the opening, which seal can be pressed against a sealing seat by the pressure of the brewing water for sealing the opening.
Such a brewing means is known from DE-PS 25 43 227. The known brewing means comprises a closing element which is displaceable on the upper side of a housing of the brewing chamber. For this purpose, an annular flange surrounding the opening of the brewing chamber is disposed on the brewing chamber, over which guides of the closing element engage. For sealing, the closing element comprises a recess open in the direction towards the brewing chamber, in which a piston plate is accommodated. The cross-sectional surface of the recess is larger than the cross-sectional surface of the opening of the brewing chamber and has approximately the same size as the piston plate. However, the thickness of the piston plate is smaller than the depth of the recess. If the closing element is in its position closing the opening, the piston plate thus rests on the annular flange around the opening of the brewing chamber, while there is a gap between the upper side of the piston plate, which faces the brewing chamber and the bottom of the recess. The hot-water line for supplying the hot brewing water ends in this gap, there being passage openings in alignment with this aperture in the piston plate, through which the hot brewing water can be guided into the brewing chamber. A sealing ring of round cross-section is embedded in such fashion in a groove in the surface area of the piston plate resting on the annular flange that it still projects beyond the surface of the piston plate. The sealing ring is disposed somewhat offset towards the center so that the surface of the piston plate located within the sealing ring is smaller than the surface resting against the gap. In this fashion the piston plate acts as a step piston, in which the seal is pressed against the annular flange acting as a sealing seat under the action of the hot water pressure. However, so that it can act as a step piston there must not remain any gap between the sealing ring and its sealing seat on the annular flange, which permits a pressure compensation at the beginning of the hot water supply across the entire lower side of the piston plate. This means that the sealing ring slides on the annular flange upon each movement of the closing element loaded by the weight of the piston plate, which has a detrimental effect on its service life and causes moreover additional friction. The projecting sealing ring also disturbs in this case, if adhering coffee powder must be removed from the lower side of the closing element. Possibly used scraping instruments can get caught and damage the sealing ring. The sealing ring can fall out and get lost. The sealing ring must furthermore be lifted above the edge of the annular flange upon the movement of the closing element, the risk of its being damaged being further increased.